Swing check valves are designed to permit flow in one direction and close automatically when the flow is reversed. Functionally, they prevent back flow in a piping system. The usual check valve is gravity actuated. The weight of the valve initiates the first stage of valve closure while complete shutoff is obtained by the fluid backflow. For each valve size and pressure range there is a minimum differential pressure through the valve at which the flow will be stable. In a case wherein a selected valve is oversized or operating at a very low flow rate, insufficient differential pressure would open the valve disc only partially, resulting in valve fluttering and banging against the valve seat, thus causing a malfunction, noisy operation and premature wear of the moving parts. Conventional check valves are not adequate in systems like steam extraction in thermal power plants and vacuum breakers in nuclear power plants where: (1) flow varies over a wide range; (2) the valve is subject to a low pressure or a low density fluid; or (3) the system requires a high level of protection from overspeeding of a turbine-generator after load rejection or tripout.
Existing construction of steam extraction check valves do not comply with design and operating recommendations to protect or at least minimize damage of large steam turbines. Major concern of engineers, consultants and power companies is that excessive overspeed of a turbine-generator shaft can be disastrous. A second factor almost as important is the energy contained in the feedwater heaters of a steam turbine-generator power cycle is often sufficient to contribute significantly to the magnitude of the turbine-generator rotating speed upon an electrical load rejection or tripout and must be prevented from doing so by means of check valves constructed in accordance with this invention.